Jet noise suppressor



June 30, 1964 E. M. DE REMER 3,139,153

JET NOISE SUPFRESSOR Filed. April 2, 1961 064? 4/. 75 PEA/5Q Q5. 5 INVENTOR.

BY m@ This invention relates generallyto reaction motors, e. g., jet engines of the types referred to, for example, in United States Patent No. 2,633,703, and those inUnited States Patent No. 2,939,279, the invention relating more particularly to the suppression of noise from such jet engines.

The invention finds its greater usefulness when embodiedin a jetengine of an aircraft. It has been determined that sound vibrations ofa frequencyabove about 15,000 cycles per second, resulting from the exhaust of an engine of a jet plane are especially annoying to people.

By this invention there is provided an assembly of two groups of tubes and a chamber between the groups, the assembly when connected to the exhaust end of a jet engine and having the dimensions of the tubes or pipes and of the chamber interrelated as is set forth more precisely hereinafter, effecting attenuation of those sound frequencies which are particularly objectionable and irritating to people.

The details of construction and dimensional relationship of component parts of two embodiments of the invention are described hereinafter with reference to the accompanying drawings in which: 7

FIGURE 1 is a longitudinal section through the exhaust end of a jet engine, and a tail pipe assembly embodying this invention;

FIGURE 2 is a cross-section through the tail pipe assembly taken upon a plane indicated in FIGURE 1 by line 22;

FIGURE 3 is a longitudinal section'through the exhaust end of a jet engine, and another embodiment of a tail pipe assembly of this invention;

. v 'FIGURE 4 is a detail section taken upon a plane indicated in FIGURE 3 by line 4-4; andv FIGURE 5 is a detail longitudinal section taken upon line 55 of FIGURE 4. V

I have found that a silencer or noise suppressor of this invention attenuates the sound of a jet engine above that frequency (about 15,000 cycles per second) which marks United States Patent 0 the upper limit tolerable by human beings. 1 In FIGURE r 1 of the drawings, reference numeral 10 designates, generally, the tail end portion of the-combustion chamber of a conventional jet engine, and numeral 12 designates a noise suppressor or tail pipe assembly of this invention connected to the exhaust end of the combustion chamber.

a The combustion chamber is defined by the engine casing 14 which in the illustrated embodiment is cylindrical and circular in cross-section, it being understood that jet engines of any conventional form, besides cylindrical, e.g., of cigar shape form, are herein contemplated. What, without the sound suppressor12, would ordinarily be the exhaust end opening for the gases jetted from the combustion chamber is designated by numeral .16.

Silencer 12 comprises a mounting plate 18, an upstream group 20 of tubes 21 extending rearwardly of the mounting plate, a plenum chamber23-defined by a casing 24 and extending downstream of the group of tubes 21, and a downstream group 26 of tubes 27 extending rearwardly from the chamber casing.

Mounting plate 18 has a peripheral flange 29 telescoped within the tail end of the jet engine casing 14, and secured as by welding or suitable fasteners not shown. Plate 18 has a plurality of openings 30 into which the forward ends of the tubes 21 of the upstream group are secured, Tubes 21 of the FIGURE 1 embodiment are cylindrical and cir- 3,139,153 Patented June 30, 1964 cular in cross-section and are arranged as close together as good design practice will permit to provide rigidity of connection of the tubes to the plate. It is, of course, obvious that tubes of hexagonal cross-section may be more closely grouped and such hexagonal configuration for the tubes 21, though not shown, is contemplated by this invention.

The casing 24 which defines plenum chamber 23 between the upstream and downstream groups of tubes cornprises a forward or front end wall 32, a rear end wall 33, and a cylindrical side wall 34. Front wall 32 has a plu rality of openings 36 into which the rear ends of the tubes 21 of the upstream group of tubes are secured. Rear wall '33 has a plurality of openings 37 into which the forward ends of the tubes 27 of the downstream group of tubes are secured. In the illustrated embodiments, the tubes 27 are equal in number and length to those of the downstream group, andthe axial dimension of the plenum chamber 23 (between front and rear walls 32 and 33) is equal to the length of the tubes. Casing 24 is preferably of a cross-section congruent with the exhaust end opening 16 0f the jet engine.

The dimensions of the groups of tubes and plenum chamber are interrelated according to the following formula:

LV 0 2 V R N wherein:

L represents the length of the tubes 21 in inches, R represents the radius of the tubes 21 in inches,

V represents the volume of plenum chamber 23 in cubic When a silencer is used, the component parts of which I r are dimensionally related in accord with the above formula, it will be found that the extent'of transmission of sound f r equencies above the cut-off frequency (f) is but about five percent of that which is transmitted without a silencer, the percent of attenuation increasingrapidly with increase of frequency up to the cut-01f frequency, and from that point leveling off to a relatively steady percent of transmission. p 0 Referring to FIGURES 3-5 there is shown another embodiment of a noise suppressor or silencer of this invention extending rearwardly from engine casing 10, this silencer being designated generally by reference numeral 46. The component parts of silencer 40 which correspond to' those of silencer 12 of the FIGURE l 'embodiment, are designated by reference numerals cone-- sponding'to thoseused forthe FIGURE. lembodiment, but in the case of the silencer 40,*the reference characters for corresponding parts are distinguished by the addition thereto of a prime. Silencer 40 comprises a mounting plate 18', a plurality of tubes 21 ofanupstream'group oftubes, a casing 24 defining a plenum chamber 23', and a plurality of tubes 27 of a downstream group of tubes. In this embodiment the number of tubes 21' in the upstream group of tubes is less than that in the silencer 12, and here again the number and length of the tubes 27 of the downstream group is equal to that of the upstream group of tubes 21.

Silencer 40 differs from silencer 12 in that in the case of silencer 40 provisions are made for the introduction of secondary air to the plenum chamber 23'. Front end wall 32' motmts a plurality of funnels 42 in its openings 36' respectively, the funnels being of frusto-conical configuration having a large end opening 44 and a small end opening 45. The funnels encircle the rear ends of the tubes 21', the tubes 21' extending coaxially Within the respective funnels. Each funnel has a spider 47 for mounting the rear end of a tube 21' concentrically within the funnel, the spider having openings 48 for flow of ram air through the funnel around the rear end of its encircled tube 21' and into the plenum chamber 23.

. As in the case of, the suppressor 12 the dimensions of the tubes 21' and the plenum chamber 23 are interrelated according to the above-mentioned'formula to effect attenuation of sound frequencies above 15,000 cycles per second, marking the cut-off point of frequencies which a are tolerable by human beings. Though in the embodiment 40, the'number and dimensions of the tubes 27' are equal to those of tubes 21', it will be understood, of course, that the" tubes 27 may be more or less in number and of larger dimensions than the tubes 21 if desired.

While the particular jet noise suppressor herein shown and disclosed in detail is fully capable of attaining the objects and providing the advantages hereinbefore stated,

it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended tothe details of construction or design herein shown other than as defined in the appended claims.

I claim:

1. In combination with the exhaust means of-a jet engine, a noise suppressor therefor, comprising an upstream group of open-ended tubes extending in a common direction and connected at their inlet ends to the rear of the combustion chamber of the jet engine, a casing defining a plenum chamber connected to the outlet ends of said tubes, and a downstream group of open-ended tubes connected to said casing whereby gases from said combustion cham ber pass through the upstream group of tubes, thence through the plenum chamber, and thence through the tubes of the downstream group, said tubes and said p-le num chamber being interrelated according to the following formula:

LV 0 2 R E wherein L represents the length of said upstream tubes in inches, R represents the radius of said upstream tubes in inches, V represents the volume of said plenum chamber in cubic inches, N represents the number of upstream tubes, 0 represents the velocity of sound in gases from said combustion chamber, and 1 represents the cut-off frequency of sound vibrations intolerable by human beings, i.e., 15,000 cycles per second.

2. The combination defined in claim 1 characterized in the provision of means for admitting secondary air to group of tubes, thence through said plenum chamber and thence through said downstream group of tubes, the downstream group of tubes being equal in number and size and dimensions to the upstream group of tubes, said tubes and said plenum chamber being interrelated according to the following formula: 4

R N* (f) wherein L represents the length of said upstream tubes in inches, R represents the radius of said upstream tubes in inches, V represents the volume of said plenum chamber in cubic inches, N represents the number of upstream tubes, c represents the velocity of sound in gases from said combustion chamber, and 1'' represents the cut-off frequency of sound vibrations intolerable by human be I ings, i.e., 15,000 cycles per second.

4. A jet engine having a noise suppressor connected to its exhaust including an upstream group of open-ended tubes extending in a common direction, said tubes having their inlet ends connected to the rear of the combustion chamber of saidjet engine, a casing defining a plenum chamber, and a downstream group of open-ended tubes, said casing having a front end wall and a rear end wall, said front end wall having a plurality of openings formed therein, a plurality of frusto-conical funnels in said openings respectively, with the smaller ends thereof being open to said plenum chamber, the upstream tubes extending at their rear ends into the funnels respectively, means for supporting the upstream 'tubes within the tunnels spaced within the funnels for allowing passage of secondary air to flow from around said upstream tubes within said funnels and into the plenum chamber, the tubes of said downstream group being connected to the rear endwall of the casing whereby gases from said combustion chamber pass through the upstream group of tubes, thence through the plenum chamber, and thence through the tubes of the downstream group, said tubes and said plenum chamber being interrelated according to the following formula:

References Cited in the file of this patent UNITED STATES PATENTS 2,037,692 Barr et al Apr. 21, 1936 2,075,316 Tyden Mar.,30, 1937 2,633,703 Tenney et a1. Apr. 7, 1953 2,795,374 Isakoff June 11, 1957 2,798,569 Fischer July 9, 1957 2,996,139 Patterson Aug. 15, 1961 FOREIGN PATENTS 151,557 Germany May 19, 1904 360,686 Great Britain Nov. 12, 1931 617,831 Germany Aug. 29, 1935 726,028.

Germany Oct. 5, 1942 

1. IN COMBINATION WITH THE EXHAUST MEANS OF A JET ENGINE, A NOISE SUPPRESSOR THEREFOR, COMPRISING AN UPSTREAM GROUP OF OPEN-ENDED TUBES EXTENDING IN A COMMON DIRECTION AND CONNECTED AT THEIR INLET ENDS TO THE REAR OF THE COMBUSTION CHAMBER OF THE JET ENGINE, A CASING DEFINING A PLENUM CHAMBER CONNECTED TO THE OUTLET ENDS OF SAID TUBES, AND A DOWNSTREAM GROUP OF OPEN-ENDED TUBES CONNECTED TO SAID CASING WHEREBY GASES FROM SAID COMBUSTION CHAMBER PASS THROUGH THE UPSTREAM GROUP OF TUBES, THENCE THROUGH THE PLENUM CHAMBER, AND THENCE THROUGH THE TUBES OF THE DOWNSTREAM GROUP, SAID TUBES AND SAID PLENUM CHAMBER BEING INTERRELATED ACCORDING TO THE FOLLOWING FORMULA: 